1. Field of the Invention
The present invention relates to a fuel injection control system for a diesel engine. More specifically, the present invention relates to a fuel injection control system used for a diesel engine having an EGR device.
2. Description of the Related Art
An EGR (Exhaust Gas Recirculation) device which recycles a part of exhaust gas of an internal combustion engine to an intake air system is widely known in the art. When EGR is applied to an engine, since a mixture of exhaust gas and air is fed to the engine, the amount of fresh air fed to the engine decreases even if the total volume of the mixture of the gases fed to the engine is the same. This causes the combustion temperature of the engine to become lower and, thereby, the amount of NO.sub.x produced by the combustion to decrease.
When EGR is applied to a diesel engine, it is required to recycle a larger amount of exhaust gas to the intake air system compared to the gasoline engine in order to obtain the above-noted EGR effect, since combustion in a diesel engine is performed at a very high excess air ratio (for example, .lambda..congruent.30).
Therefore, in some cases, a throttle valve disposed on the intake air passage of the diesel engine is used in order to recycle a large amount of exhaust gas. By throttling the intake air passage by the throttle valve, the amount of intake air (fresh air) flowing into the intake air passage decreases and the pressure in the intake air passage downstream of the throttle valve becomes low. Therefore, in this case, the amount of exhaust gas fed to the intake air passage can be largely increased by feeding exhaust gas to the intake air passage downstream of the throttle valve.
An example of an EGR device utilizing a throttle valve is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-143343. The throttle valve in the '343 publication is connected to an accelerator pedal of the diesel engine and the degree of opening thereof is adjusted in accordance with the accelerator stroke (i.e., the amount of depression of the accelerator pedal).
In the '343 publication, the target value of the fuel injection amount is set in accordance with the accelerator stroke and the engine speed. Further, the device in the '343 publication determines whether the engine is accelerating based on the amount of change in the accelerator stroke, and if the engine is accelerating, the device terminates EGR (i.e., the feed of the exhaust gas to the intake air passage) and restricts the fuel injection amount so that the fuel injection amount does not exceed a maximum fuel injection amount determined by the engine speed and the boost pressure of the engine. Namely, in the '343 publication, EGR is stopped and the actual fuel injection amount is restricted to a maximum fuel injection amount when the engine is accelerating. When the fuel injection amount is large compared to the amount of intake air fed to the engine, exhaust smoke is generated by the combustion in the engine. The '343 publication intends to suppress the generation of this exhaust smoke by terminating EGR and limiting the fuel injection amount to the maximum fuel injection amount during acceleration.
However, it is found that, in some cases, the exhaust smoke is still generated during acceleration if the maximum fuel injection amount is determined only based on the engine speed and the engine boost pressure as in the device of the '343 publication. Though the device in the '343 publication stops EGR when acceleration is detected, the feed of the exhaust gas to the engine does not stop immediately due to a delay in the response of the EGR device and the exhaust gas already fed to the intake air passage. Therefore, exhaust gas is continuously fed to the engine even after EGR is stopped until the EGR device responds to the stop signal and all the exhaust gas remaining in the intake air passage is drawn into the engine. Thus, in the '343 publication, the amount of intake air actually fed to the engine immediately after EGR is stopped is smaller than the amount of intake air where no exhaust gas is drawn into the engine even if the engine speed and the boost pressure are the same. As explained before, the limit value of the fuel injection amount (the maximum fuel injection amount) in the '343 publication is determined from the engine speed and the boost pressure. Therefore, the limit value is set at a value excessively large compared to the actual amount of intake air immediately after EGR is stopped during acceleration. Thus, in the '343 publication, the exhaust smoke is generated during acceleration.